1. Field of the Invention
The present invention relates to a driving device for a vibration type motor which utilizes a resonance of a vibration member.
2. Related Background Art
In recent years, a vibration type motor called an ultrasonic wave motor, a piezoelectric motor, or a vibration wave motor has been developed, and has been put into practical applications by the present applicants. As is well known, the vibration type motor is a non-electromagnetic driven type motor, in which AC voltages are applied to electro-mechanical energy conversion elements such as piezoelectric elements or electrostrictive elements to cause such elements to generate a high-frequency vibration, and the vibration energy is picked up as a continuous mechanical motion.
FIG. 13 is a side view which shows a conventional bar-shape ultrasonic wave motor, and also shows the arrangement of wiring lines for supplying voltages to piezoelectric elements arranged in the motor, and for extracting an output voltage therefrom. A vibration member 1 constitutes the bar-shape ultrasonic wave motor, and comprises a coupled structure of piezoelectric or electrostrictive elements and an elastic member.
The piezoelectric element portion of the vibration member 1 is constituted by A- and B-phase driving piezoelectric elements a1, a2, b1, and b2, and a vibration detection piezoelectric element S. When an A-phase applied voltage is applied to a portion sandwiched between the A-phase piezoelectric elements a1 and a2, and a B-phase applied voltage is applied to a portion sandwiched between the B-phase piezoelectric elements b1 and b2, the piezoelectric elements are driven. Also, the rear sides of the A- and B-phase piezoelectric elements a1, a2, b1, and b2 are connected to the GND potential. One surface of the vibration detection piezoelectric element S is similarly connected to the GND potential, and a signal is output from the other surface thereof. The signal output surface of the vibration detection piezoelectric element S contacts a metal block. The block is insulated from the GND potential by an insulation sheet. Therefore, the vibration detection piezoelectric element S can directly output an electric power voltage corresponding to a vibration generated therein. A resonance frequency or the like is calculated on the basis of the magnitude of the output voltage or its phase difference from a driving voltage.
FIG. 14 shows a driving circuit for such a vibration wave motor. The driving circuit comprises driving electrodes A and B for applying AC voltages to the piezoelectric or electrostrictive elements, an oscillator 2 for generating an AC voltage, a 90.degree. phase shifter 3, switching circuits 4A and 5B for switching AC voltages from the oscillator and the phase shifter by a power supply voltage, and booster coils 6 and 7 for amplifying pulse voltages switched by the switching circuits 4A and 5B. The driving circuit also includes a phase difference detector 8 for detecting a signal phase difference between the driving electrode A and a vibration detection electrode S.
The driving circuit further includes a control microcomputer 10. FIG. 15 is a waveform chart showing signals from the driving electrode A and the vibration detection electrode S shown in FIG. 14.
The control microcomputer 10 supplies a command to the oscillator 2 to generate an AC voltage having a given frequency at which the vibration wave motor is to be driven. The signals output from the driving electrode A and the vibration detection electrode S have regular sine waveforms, as shown in FIG. 15. Therefore, the phase difference detector 8 can output a signal corresponding to the phase difference at that time to the microcomputer 10. The microcomputer 10 detects a current difference from a resonance frequency on the basis of the input signal, and controls a drive of the motor at an optimal frequency. In this manner, the driving frequency can be controlled.
When such a bar-shape vibration wave motor includes an odd number of driving piezoelectric elements, as shown in FIG. 16, a driving voltage is undesirably applied to a vibration member portion, and an appropriate detection output cannot be obtained even if a vibration detection piezoelectric element is simply stacked on the driving piezoelectric elements.
Also, a problem of an increase in driving voltage is posed since the vibration wave motor uses piezoelectric elements. As a countermeasure against this problem, a method which adopts a floating structure shown in FIG. 17 to halve the conventional driving voltage has been proposed.
FIG. 18 shows a driving circuit for such a vibration wave motor. The driving circuit comprises driving electrodes A, A', B, and B' for applying AC voltages to the piezoelectric or electrostrictive elements, an oscillator 2 for generating an AC voltage, a 90.degree. phase shifter 3, switching circuits 4A, 4A', 5B, and 5B' for switching AC voltages from the oscillator and the phase shifter by a power supply voltage, and booster coils 6 and 7 for amplifying pulse voltages switched by the switching circuits 4A, 4A', 5B, and 5B'.
The driving circuit also includes a control microcomputer 10. The control microcomputer 10 supplies a command to the oscillator 2 to generate an AC voltage having a given frequency at which the vibration wave motor is to be driven. At this time, the switching circuits 4A, 4A' having a 180.degree. phase difference to applied voltages therebetween, and 5B, 5B' also having a 180.degree. phase difference of applied voltages therebetween, switch input signals to electrodes A, A' or B, B' at the input timings. In this case, a voltage twice the power supply voltage is apparently applied to the driving electrodes A, A', B, and B' via the coils. Therefore, the piezoelectric elements can be driven by a voltage half that required in the conventional motor. However, even when the floating structure is adopted, the above-mentioned problem is posed upon arrangement of the detection piezoelectric element.
As described above, in the vibration type motor structure shown in FIG. 16 or 17, even when the vibration detection piezoelectric element S is simply stacked on the driving piezoelectric elements as in the conventional method, a vibration cannot be appropriately detected.